1. Field of the Invention
The present invention relates generally to Imagers, and to a method of fabrication thereof.
2. Description of the Related Art
Visible light cameras contain optics that focus light to form a visible image on a focal plane. In an electronic camera, the image is converted into an analog or digital electrical signal by an electronic sensor chip.
The light sensitive area of the electronic sensor chips is divided into an array of picture elements, or pixels. Light falling on a pixel liberates electric charge which is then converted in to a signal voltage, which is proportional to the intensity of the light incident on that pixel. The signal voltage for each pixel is then rapidly read out in turn, producing a video or video-like signal that is representative of the image focused on the electronic sensor chip.
Monolithic sensor chips made out of semiconducting silicon are widely used as visible light sensors, for applications such as cell phone cameras and digital still cameras. The term “monolithic” means that the sensor chip is made from a single piece of silicon. In a monolithic imager, the light-sensitive photodiode is integrated into the same silicon as all the other electronics. Each photodiode is surrounded by several transistors that buffer the signal produced by the photodiode, and also allow the resetting and selection of the photodiode.
Together these transistors, and the integrated photodiode, form a pixel. An imaging array of a quarter of a million, to many million pixels, form the light sensitive part of the chip. The imaging array on the chip is then surrounded by signal processing and control electronics.
The silicon in a monolithic imager is typically several hundred micrometers thick, but only the top few micrometers is optically sensitive, with the remaining silicon substrate simply acting as a mechanical support and electrical ground plane. The light is brought in from the front, and passes through gaps in the metallization and transistor layers before it is absorbed in the silicon, liberating signal charge.
Some monolithic imagers are back-illuminated. Here, the imager is mounted such that it is mechanically supported from the front, and the thick, optically insensitive substrate is mechanically and chemically removed, leaving the back surface of the thin, optically sensitive silicon exposed. The imager is mounted with the back surface out, so that light is incident on the exposed back surface. Signal charge liberated by the light diffuses or drifts to the front surface where it is collected and converted to a signal voltage.
Visible hybrid imagers are sometimes used for scientific applications. Hybrid imagers consist of two chips connected together. Typically, one chip is the Detector Array, and it consists of an array of photodiodes only (with no transistors). The other chip is the Readout Integrated Circuit (ROIC), and it contains the pixel transistors, as well as all of the surrounding support electronics. The chips are arranged so that the electronics on each chip (the photodiodes on the Detector Array and the transistors on the ROIC) face each other. The diodes are then connected to the pixel transistors through a chip-to-chip interconnect, typically an indium “bump” on each chip that is cold welded together when the chips are forced together. This requires on interconnect for each pixel, or one million interconnects for a one million pixel array. The architecture for a hybrid visible imager 100 is illustrated in FIG. 1.
FIG. 1 illustrates a conventional hybrid imager architecture 100, wherein the Detector Array 102 chip contains only photodiodes 104 and no transistors 106, and the pixel transistors 106 are on the corresponding location on the ROIC 108 (which also contains the control and signal handling electronics, such as analog-to-digital converters (ADCs)). The transistors comprise buffer 110, select 112, and reset 114 transistors.
Because the electronics side of the detector array must face the ROIC, their front surface is covered. Therefore, hybrid imagers can only be back illuminated. The detector array can be thinned, as described above for monolithic imagers.
Alternatively, the detector array can be made from very lightly doped silicon, which allows optically generated charge to drift or diffuse through the thick silicon without recombining Detector arrays made from very lightly doped silicon can therefore be thick but still have good response.
All of these detector arrays have the control or signal handling electronics in the same plane as the pixel electronics, so they must surround the imaging array. This produces a “dead zone” of optically insensitive silicon if one attempts to make a large mosaic array by tiling smaller, individual chips.
In addition, hybrid arrays require extremely large numbers of interconnects.